Landscaping on a Slope: A How-To Guide

Slopes are one of the most common landscaping problems, particularly
in houses above street level. Although a gentle slope
(two or three degrees) can help direct water from buildings,
and can be landscaped as a flat area would be, a slope of 4 percent
or more requires special treatment. Here are some design ideas and plant choices to improve the slope in your garden.

Problems (or why you should landscape a slope)

A slope of four degrees or more (usually more than one foot
difference) can cause several problems. If planted with turfgrass, it can be difficult and dangerous to mow. Low water infiltration rate means the top of the hill ends up dry while plants at the foot of the hill become water-logged. Fertilizer will wash down the hill, meaning plants at the top get no nutrition and plants at the bottom are poisoned by the build-up of nutrients. Needless to say, that makes it hard to grow plants in either location. Slopes can also be difficult to mulch because lighter organic mulch ends up rolling down.

But there are plenty of things you can do about it…

Solutions

The simplest solution is to replant the slope with groundcovers,
ornamental grasses, or other plants that grow well on inclines. If you’ve got turf growing there already, just remove it and re-plant the area with your groundcover of choice. For steeper slopes, though, you can let the turf die down, cut holes in to place new plants in, and leaving the turf as mulch. Doing this makes it easier to get the plants established. If you prefer grass and the slope
isn’t too steep (not over five degrees) buffalograss or fine-leaf fescues
are some other planting choice to try.

Looking for plants that grow well on a slope? Check out the selection of ground covers at Michigan Bulb. Some good picks for foliage-only ground covers are fescue, English ivy, and pachysandra (pachysandra’s great for providing attractive greenery year-round). If you’d prefer a flowering ground cover, try creeping myrtle or one of the many varieties of phlox. (See the end of this article for more plant suggestions.) All these are available from Michigan Bulb.

If the site you’re working with is surrounded by a flat area, removing the turf, smoothing out the soil, and setting the turf in again (or replacing it with
new, purchased turf) may be enough.

Terracing

For steeper hills, or those in areas that can’t be re-graded easily, terracing will solve the slope problems by creating smaller, level planting beds. Make level
steps up the hill using railway ties (use these around non-edible plants only), boulders, or other building material to create the walls going up the hill. The flat
areas can then either be used as individual beds or planted with a
low-maintenance, cascading groundcover.

Retaining walls

Another option for steeper hills is a retaining wall at
the base of the slope. Behind the wall will be an area of
well-drained soil that should be ideal for planting. However, when
choosing plants for the area, consider which direction
the sun comes in during the summer months—will the wall be blocking sun
and creating a pool of shade, or will it be trapping sun and creating a hot, dry area?

Slope maintenance

Due to the low infiltration rate, correctly watering a slope can
be tricky. Dividing the watering time into two sessions will give time
for the water to be absorbed. To determine how long each watering
should last, water the slope and keep track of the time
until run-off occurs at the bottom of the slope. Wait several hours for the
water to absorb, then repeat this process (timing again)
until the root areas have received enough water. Because water
will run down the slope,
watering the top only, not the middle and bottom, will suffice.
Aerating the top of the hill with a spiked aeration tool
and adding water-holding
compost or processed clay can also improve infiltration.
Instead of organic mulch, which has a tendency to roll and drift downhill, try river rock, washed stone or commercially produced colored rubber tire chunks.

Plants for a slope or hill

Typically, the best plants for a slope are ground covers that send out roots
at intervals along their stems, those with many small stems from one base (clumping plants), and spreading plants. These tend to do the best job of holding soil in place.

For gentle slopes grasses may be enough. Fescues such as sheep fescue
(Festuca ovina), blue fescue (Festuca ovina glauca), or creeping red
fescue (Festuca rubra) work well. These usually form tufts rather than smooth, lawn-like growth and they do become rather tall, but they’re otherwise attractive and interesting additions to the landscape. Slow-growing buffalograss, which rarely reaches over 8 to 9 inches, is another possible grass.

On steeper slopes, however, grasses will wash away.
Instead, opt for one of the many plants that don’t mind growing
on an incline. These include English ivy (Hedera helix), Hall’s honeysuckle (Lonicera japonica
‘halliana’), purple-leaved wintercreeper (Euonymus fortunei ‘Coloratus’), French lavender (Lavandula stoechas), periwinkle (Vinca minor), rosemary,
yarrow, lamb’s ear (Stachys bysantina), lily-of-the-valley (Convallaria majalis), gazania,
rockspray cotoneaster (Cotoneaster horizontalis), Seaside daisy (Erigeron glaucus), and Swan River daisy (Brachyscome
iberidifolia), cranesbill geranium (Geranium sanguineum), and—one of the most common choices—junipers. Vetch is sometimes recommended, but in most areas it acts as a fast-spreading invasive weed and is best avoided.